This invention is directed to a control system for use with a catalytic converter for reducing undesirable substances in the exhaust gases of internal combustion engines.
It is well known that exhaust gases from an internal combustion engine can be catalytically treated to reduce the amounts of hydrocarbons, carbon monoxide and oxides of nitrogen, the catalytic treatment including oxidation of carbon monoxide and hydrocarbons and reduction of nitrogen oxides.
A single catalytic device may be utilized to accomplish both the oxidation and reduction necessary for minimizing the undesirable exhaust components provided that the air-fuel mixture supplied to the catalytic converter is maintained within a narrow range (hereinafter referred to as the converter window) at stoichiometry, the ratio containing fuel and oxygen in such proportions that, in perfect combustion, both would be completely consumed. Numerous fuel control systems have been suggested in which the air-fuel ratio of the mixture supplied to the internal combustion engine is controlled by feedback from an exhaust sensor for maintaining the gases supplied to the converter within the converter window. One such system is described in U.S. Pat. No. 3,939,654 issued on Feb. 24, 1976, and which is assigned to the assignee of this invention, the contents of which is hereby incorporated by reference. As described in this patent, two zirconia sensors are utilized in a control system wherein the first zirconia sensor is exposed to the exhaust gases upstream from a catalytic converter and a second zirconia sensor is exposed to the exhaust gases downstream from the catalytic converter. The signals from the zirconia sensors are combined and fed back through appropriate control elements to vary the air-fuel ratio of the engine mixture in order to maintain the air-fuel ratio of the mixture supplied to the catalytic converter within the converter window to optimize the oxidation and reduction necessary to minimize the undesirable exhaust constituents. The zirconia sensor downstream from the catalytic converter exhibits sharper sensitivity to a change in air-fuel ratio and provides a signal which maintains the system within the converter window over time without drift and the zirconia sensor upstream from the catalytic converter provides a quicker response, since it does not involve the time delay introduced by the catalytic converter, to reduce transient swings out of the converter window and helps reduce the required gain in the feedback loop to improve stability of the system.
In the system using the two zirconia sensors, during initial operation of the vehicle engine, the first zirconia sensor upstream from the catalytic converter is quickly heated to its operating temperature at which it provides an output voltage representing the air-fuel ratio of the exhaust gases upstream from the catalytic converter. However, the zirconia sensor downstream of the catalytic converter is not heated to its operating temperature for a time delay which is imposed by the catalytic converter. For this time period, the zirconia sensor downstream of the catalytic converter is inoperative to produce a useable voltage signal representative of the air-fuel ratio of the exhaust gases downstream of the catalytic converter. This would normally command a rich air-fuel mixture. However, during this catalytic converter imposed time delay, the control system operates to control the mixture of air and fuel to the internal combustion engine to the lean side of stoichiometry. After the zirconia sensor downstream of the catalytic converter is heated to its operating temperature, it is then effective to supply a voltage indicating the air-fuel ratio so that the control system functions to control the air-fuel mixture supplied to the engine at stoichiometry.